1. Field of the Invention
The present invention relates to a flux and method for joining dissimilar metals, and more particularly, to a method for joining dissimilar metals using a borax-based flux.
2. Background of the Related Art
Generally, a cylinder block, which is used in a hydraulic motor and pump, rotates on a valve plate, a counter-friction material, at high speed and high pressure, to form hydraulic pressure, thus obtaining kinetic energy or converting kinetic energy to hydraulic energy. In this regard, friction occurs between the cylinder block rotating at high speed and high pressure and the valve plate and causes energy loss and the mechanical abrasion of each element. For this reason, it is needed to minimize the friction.
It is occasionally needed to join dissimilar metals according to the requirements for engine bushings or pipes requiring oxidation resistance. Accordingly, various methods for joining the dissimilar metals are being developed.
Regarding these methods, Korean patent publication No. 10-1993-7666 discloses a liquid phase diffusion joining method including the use of insert materials having a higher melting temperature than that of a base metal. The method disclosed in this publication uses the diffusion reaction between the insert materials and the base material, in which the insert materials have a purity of 95% and are selected from the group consisting of B, C, Hf and Si, and the base material is selected from the group consisting of Ni, Co, Fe and Ti.
Korean patent registration No. 10-0205160 discloses a joining part between Ni—Ti alloy and a dissimilar metal, and a method for forming the same. The method disclosed in this patent comprises bring the joining surfaces of the Ni—Ti alloy component and the dissimilar metal component into contact, heating the contacted metal components to the fusion temperature of either of the two metal components so as to fuse the two components by local reaction, softening the metal components at high temperature, and applying high pressure to both the metal components to form a fusion-joined texture at the joining interface.
Korean patent publication Nos. 10-1996-15802 and 10-0143744 disclose methods for joining dissimilar metals to a hollow cylinder. One method disclosed in these patents comprises placing a joining metal in the hallow of a base metal, filling the hollow of the joining metal with a thermal insulation material for making directional solidification, heating the resulting structure in a general atmosphere or a vacuum furnace to a temperature lower than the melting point of the base metal but higher than the melting temperature of the joining metal, and cooling the heated metals. Another method disclosed in the patents includes the use of any one of Zn and P as an additive.
Korean patent registration No. 10-0136954 discloses a joining method between dissimilar metals, such as aluminum-stainless steel, or between dissimilar materials, such as metal-ceramic. The method disclosed in this publication comprises vacuum-depositing an active metal evaporation material on the joining surface of a base material, penetrating and diffusing the active metal deposition layer into the base material, inserting a filler metal between the two materials, and thermally treating the resulting structure at 900–950° C. for 10–50 minutes under a vacuum atmosphere.
Korean patent laid-open publication No. 10-2000-46439 discloses a method for joining dissimilar materials, which comprises kneading hard powder and brazing filler metal into one flexible mat, and joining the flexible mat to a base metal using a polymer binder. Korean patent laid-open publication No. 10-2001-61826 discloses a method comprising forming copper alloy powder into a liner material having a given shape, laying the liner material on a base material, and pressing and sintering the resulting structure at a temperature of 650–900° C. under a pressure of 0.1–40 kgf/cm2.
Korean patent laid-open publication No. 10-2002-26768 discloses a method for joining dissimilar metal plates, which comprises laying a non-ferrous alloy plate on a ferrous alloy element, applying a given pressure to the two materials at a lower temperature than their melting point so as to enhance the adhesion therebetween, and thermally treating the resulting structure at a higher temperature than the recrystallization temperature thereof under a reducing or non-oxidizing atmosphere. Korean patent registration No. 10-0479486 discloses a method comprising joining a liner to a base material in a heating furnace at high temperature. Korean Patent registration No. 10-039300 discloses a method for joining dissimilar metals, which comprises applying copper alloy powder on a base metal, pressing the two metals, sintering the resulting structure at a temperature of 880–980° C., followed by slow cooling.
However, the prior methods as described above have many problems in actual use, in that a separate thermal treatment process is performed or a separate element is made.
In an attempt to solve such problems, Korean patent application No. 10-2004-5718 filed in the name of the applicant discloses a method for joining dissimilar metals, which comprises mixing a flux with metal powder, coating the mixture on a metal to be joined, and preheating the coated metal. However, this method is complicated in that the mixture of the flux and the metal powder is made and the preheating process is performed.